After upgrade, Maintenance Tool fails to connect via proxy

I ran an update on the Maintenance Tool and it upgraded itself to the latest version. After restarting the tool (no other changes were made) the Maintenance tool will no longer connect, reporting "Failed to connect to server, check your network" (in red letters).
This system only has proxy access to the internet. No direct connectivity.
I have set the manual proxy settings in the settings screen. I have also tried removing the settings and putting them back in.
The proxy server logs show no sign of any attempted connection (allowed, denied or otherwise) to any qt.io site. (However, the proxy logs do clearly show the connections made to upgrade the Maintenance Tool itself. ie. it worked fine until this last upgrade ran today).